Frequency selective electrical signal filters for communications applications were developed beginning around 1910, for telegraphy and telephony uses, particularly for multiplexing and de-multiplexing of communication signal channels carried on long distance cables and wireless links. Filter design methods, named “image” or “image parameter” design methods were developed by Bell Laboratories, among others, in the 1920s (see George A. Campbell, Physical Theory of the Electric Wave Filter, The Bell System Technical Journal, Volume I, No. 2 (November 1922); Otto J. Zobel, Theory and Design of Uniform and Composite Electric Wave-Filters, The Bell System Technical Journal, Volume II, No. 1 (January 1923)). Using these techniques, the filter is designed as a transmission line that is topologically broken up into often identical sections that have the same or similar input impedance, and the same or similar output impedance. The sections are connected in alternating fashion so that the inputs of adjacent sections connect to each other, and the outputs of adjacent sections connect to each other (i.e., the input of the first section is connected to the input of the second section, the output of the second section is connected to the output of the third section, the input of the third section is connected to the input of the fourth section, etc.). Since the input impedances or output impedances always face each other, there will be no reflection at the interfaces between the sections as a signal is transmitted through the filter.
Generally, the image design method produces an “initial filter design”. More design steps are needed to produce a “final filter design” that can be manufactured. These additional steps may include: combining like adjacent elements, adding or deleting specific circuit elements to produce a desired enhancement to the filter characteristic, adding parasitic effects not included in the idealized circuit element models to more accurately represent the physical circuit to be manufactured, performing a computer optimization of the circuit element values to better match the desired requirement, etc.
Acoustic wave (AW) resonators, specifically quartz bulk acoustic wave (BAW) resonators, began to be used in some electrical signal filters. The equivalent circuit of an AW resonator has two resonances closely spaced in frequency call the “resonance” frequency and the “anti-resonance” frequency (see K. S. Van Dyke, Piezo-Electric Resonator and its Equivalent Network Proc. IRE, Vol. 16, 1928, pp. 742-764). The image filter design methods were applied to filter circuits utilizing these quartz resonators, and two AW filter circuit types resulted: “ladder” and “lattice” AW filter designs (see U.S. Pat. No. 1,795,204; W. P. Mason, Electrical Wave Filters Employing Quartz Crystals as Elements, The Bell System Technical Journal (1934)). In subsequent decades, the quartz ladder design was typically only used for single channel filters due to its extremely narrow bandwidth. The majority of quartz filters were hybrid-lattice designs, which allow less narrow bandwidths, but normally require inductors.
Network synthesis designs began to appear in the 1960s, which permitted a much wider variety of filter circuit designs, but also normally required inductors, which tend to be physically large and lossy compared to capacitors. These designs were at RF frequencies and lower (<100 MHz) and were made using bulk crystals, often quartz. Surface acoustic wave (SAW) filters also began to appear at this time. These designs suffered from high insertion losses due to transducer losses, allowing uses only at intermediate frequencies—not radio frequencies, and were based on transversal designs, also termed “tapped delay lines.”
Beginning in about 1992, thin film SAW resonators and BAW resonators were developed and began to be used in microwave (frequencies >500 MHz). AW impedance element filter (IEF) designs, which can also be referred to as Espenschied-type ladder acoustic wave filter designs (see O. Ikata, et al., Development of Low-Loss Bandpass Filters Using Saw Resonators for Portable Telephones, 1992 Ultrasonics Symposium, pp. 111-115). Image designed AW IEF bandpass filters in SAW and BAW implementations are often used for microwave filtering applications in the radio frequency (RF) front end of mobile communications devices. Of most particular importance in the mobile communication industry is the frequency range from approximately 500-3,500 MHz. In the United States, there are a number of standard bands used for cellular communications. These include Band 2 (˜1800-1900 MHz), Band 4 (˜1700-2100 MHz), Band 5 (˜800-900 MHz), Band 13 (˜700-800 MHz), and Band 17 (˜700-800 MHz); with other bands emerging.
The duplexer, a specialized kind of filter is a key component in the front end of mobile devices. Modern mobile communications devices transmit and receive at the same time (using LTE, WCDMA or CDMA) and use the same antenna. The duplexer separates the transmit signal, which can be up to 0.5 Watt power, from the receive signal, which can be as low as a pico-Watt. The transmit and receive signals are modulated on carriers at different frequencies allowing the duplexer to select them, even so the duplexer must provide the frequency selection, isolation and low insertion loss in a very small size often only about two millimeters square. The image designed bandpass AW IEF filter is universally preferred to be used in a duplexer, because it satisfies these requirements, and significantly better than alternatives like the tapped delay line (since it has higher loss), and the resonant single-phase unidirectional tranducer (SPUDT) filter (since the narrow lines required prevent scaling to microwave frequencies); although the double-mode SAW (DMS) (also called longitudinally coupled resonator (LCR)) filter is sometimes used for the receive filter in a duplexer due to the balanced output it provides. (See David Morgan, Surface Acoustic Wave Filters With Applications to Electronic Communications and Signal Processing Morgan, pp. 335-339, 352-354 (2007)). Traditionally, the IEF filters utilize a simple paired resonator architecture consisting of only one in-line resonator and only one in-shunt resonator for each image section.
Minor variations to these traditional AW IEF filter designs have also been considered for these applications (see U.S. Pat. No. 8,026,776 and U.S. Pat. No. 8,063,717), which typically add one or more circuit elements (e.g. capacitor, inductor, or AW resonator) to the IEF design to enhance a particular circuit feature. This can be accomplished when the influences to the basic AW IEF circuit are minor enough that common computer optimization tools converge to produce an improved design versus the traditional AW IEF filter design. This is a stringent requirement for any circuit containing closely spaced resonances and anti-resonances, like the resonators used in an AW IEF filter, and thus permits only very minor variations to the basic AW IEF design and function. This is because a primary requirement for computer circuit optimization routines to converge to an improved circuit design solution is that the initial design be the same circuit structure as the final improved design and that the initial circuit element values be very close to the final values. Thus, the basic architecture of AW IEF filter designs has been limited to the simple paired resonator architecture and minor alterations to the basic AW ladder design, alterations made “after the fact” to this traditional circuit design. There is a need for improved microwave AW filters in terms of cost, loss, size and power handling for mobile communications.